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Volcano Watch — What was that on the Richter scale?

February 21, 2008

When the crust of the earth is subjected to great forces, such as the weight of the Hawaiian Islands or the forceful intrusion of magma, it can bend, compress, or stretch.

A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the earth's surface (Rayleigh and Love waves) or through the earth's interior (P and S waves).

Because the Earth's crust is elastic, it wants to bounce back into its original position. If these applied forces are greater than the strength of the rocks that make up the Earth's crust, then there may be a sudden breaking and movement within the crust, thereby releasing the built-up energy.

The released energy radiates, much like ripples from a stone thrown into a pond, as seismic waves ("seismos" is a Greek word meaning "earthquake"). These seismic waves cause the ground motion we know as earthquakes.

By studying these seismic waves, seismologists can determine a number of things about an earthquake, including the location and magnitude. The location of an earthquake can be determined by measuring the relative arrival times of seismic waves at several seismic recording stations. An earthquake's magnitude can be a much more complex calculation.

The notion of a magnitude scale for earthquakes was developed by Dr. Charles F. Richter, who recognized that the measured amplitude of a seismic wave (the size of the wiggle recorded on a seismogram) provides a good estimate of the energy released by an earthquake. He developed the Richter scale, a relationship between the logarithm of the measured amplitude to the earthquake's magnitude for southern California.

Although modern earthquake magnitude scales are popularly known as Richter scales, the actual formula that Richter developed is directly applicable only to southern California, using a specific type of recording instrumentation.

There are several types of magnitude scales in use today (see, and each should provide similar magnitudes for the same earthquake. The duration magnitude (Md) is a measure of how long the ground keeps shaking after an earthquake happens. The local magnitude (ML) is closest to the Richter magnitude and is determined from amplitudes of seismic waves recorded within about 350 miles of an earthquake. If an earthquake is large enough, the body wave magnitude (Mb) can be determined, using seismic waves that have traveled through the earth to be recorded globally. The moment magnitude (Mw) is directly related to the size of the earthquake rupture zone and the total energy released in the earthquake.

Since these magnitude scales are all logarithmic, a whole number increase in magnitude represents a 10-fold jump in measured amplitude, for example, a magnitude 4.0 has 10 times the measured amplitude of a magnitude 3.0. The total energy released from an earthquake increases even more rapidly with magnitude. A whole number increase in magnitude represents a jump in released energy of about 31.6 times. A magnitude increase of two whole numbers represents an increase in released energy of 1,000 times.

It can be difficult to visualize such a large range in levels of energy but we can relate the energy levels to other natural and man-made phenomena.

For example, a magnitude 1.0 earthquake is roughly equivalent to the energy release from an explosion of about 70 pounds of TNT (a mid-sized construction site blast), a magnitude 2.0 earthquake is similar to an explosion of 1 metric ton of TNT, and a magnitude 4.0 earthquake is approximately equivalent to the energy release from an explosion of 1,000 tons of TNT (a small nuclear blast).

Other examples include the 1980 Mount St. Helens eruption, which released the energy equivalent of a magnitude 7.8 earthquake (or just over 500 megatons of TNT), and the 1883 Krakatoa eruption, which released the energy equivalent of a magnitude 8.5 earthquake (or about 5.6 gigatons of TNT).

Earthquake magnitude scales allow us to quantify the amount of energy released in an earthquake and compare one earthquake with another, as well as with other natural and man-made phenomena. And, thanks to Charles Richter, they give us a nice, small number to report on the news after an earthquake.


Volcano Activity Update

Lava from the 2007 Thanksgiving Eve Breakout (TEB) flow, erupting from fissure D of the July 21st eruption, continues to slowly advance toward the southeast near the top of the Royal Gardens subdivision. This multi-fingered pahoehoe flow is being fed from the end of the rootless shield complex constructed southeast of the TEB vent since November. While most of the active lava remains above the pali, a narrow finger had crept into the top of the subdivision on older flows by Wednesday, February 20.

An area of persistent breakouts on the northeast side of the shield complex also continues to produce small flows. These northeast-directed flows are not advancing and are restricted to a broad, flat area on the south side of Kupaianaha.

Weak incandescence has been observed in Pu`u `O`o almost nightly for the last several days. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is briefly stored and substantially degassed on its way to the eruption site.

Mauna Loa is not erupting. One earthquake was located beneath the summit in the past week. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.

Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-3.6 earthquake at 11:30 a.m., H.s.t., on Friday, February 15, was located 6 km (4 miles) north-northeast of Ka`ena Point at a depth of 9 km (6 miles). A magnitude-3.9 earthquake at 6:14 p.m., H.s.t., on Tuesday, February 19, was located 10 km (6 miles) west-northwest of Ka`ena Point at a depth of 10 km (6 miles).

Visit our Web site for daily Kīlauea eruption updates and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862. skip past bottom navigational bar.

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